(1) Field of the Invention
The present invention relates to the generation of sound and particularly to the projection of a modulated column of air which may function as an audible warning. More specifically, this invention is directed to a siren system and especially to a siren system which may be installed in vehicles where space is severely limited. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well-suited for use in siren systems designed for mounting in emergency vehicles. In the interest of fuel economy, many public safety organizations have adopted the use of vehicles generally described as "compact" or "sub-compact" by their manufacturers. While the employment of an audible attention attracting device, i.e., a siren, on such emergency vehicles is mandated, the desire to reduce wind resistance and enhance safety by removing externally mounted irregularly shaped objects dictates that the siren be positioned under the vehicle hood such that the sound produced thereby will be transmitted through the cooling air intake space. As is well known, mounting space is limited in the engine compartment of most modern vehicles and such space is severely limited in compact and sub-compact vehicles. It is equally well known that previously available siren systems were rather bulky devices, i.e., were characterized by volumetric inefficiency. This volumetric inefficiency has previously precluded the under-the-hood mounting of sirens on compact and sub-compact vehicles.
The volumetric efficiency of a siren for use on an emergency vehicle may be enhanced through the use of an "electronic" siren. A electronic siren includes a speaker/driver unit which comprises a compression driver, i.e., a loud speaker capable of high output power which typically employs a phenolic diaphragm. The diaphragm is driven, in the known manner, by the modulated output signal of a power amplifier which may be positioned remotely with respect to the driver unit. A re-entrant horn is mounted on the driver unit whereby, in the manner known in the art, the sound produced by the moving diaphragm will be amplified and directed. The re-entrant horn is typically defined by a speaker bell which has, centrally mounted therein, a speaker cone. The air which is set into motion by the driver unit diaphragm is forced through a throat, i.e., a reduced diameter output portion of the driver unit, and discharges into the cone. The re-entrant horn and bell cooperate to define a relatively long path length along which the cross-sectional area smoothly increases whereby an amplified sound will be projected in a direction which is generally co-axial with the throat of the drive unit. The combination of the driver sub-assembly and re-entrant horn mounted thereon is relatively large.